Humans caused climate change and the loss of many animal species – a study now shows how fatal the combination can be: Without the control of sea otters and with the support of climate change, armies of sea urchins are now increasingly devouring the ecologically important calcareous algae reefs of Alaska. According to the results, it was only the combination of both factors that led to a tipping point being exceeded. There is hope, however: the restoration of the sea otter populations could possibly stabilize the ecosystem again, say the researchers.
The coral reefs of the tropical seas are known for their biodiversity and their far-reaching importance for the creatures of the oceans. But cool sea regions also have something similar to offer. In the area of the Aleutian Islands, the calcareous red alga Clathromorphum nereostratum forms complex structures on the sea floor that provide a habitat for many organisms and also serve as a base for the seaweed forests of the coastal areas. Sea urchins are a natural predator of the reef-building algae. To protect themselves from them, the algae produce skeletons from lime. But the sea urchins can only be prevented from doing so to a limited extent – they can drill their way through the protective layer.
Destroyed equilibria
For a long time the system was in equilibrium as sea urchin populations were kept in check by sea otters. The population of these cute predators has shrunk so much due to the influence of humans that they can no longer fulfill this function in the ecosystem. This led to an unnaturally high population of hungry sea urchins. First they ate the offspring of the seaweed and thus heavily thinned the kelp forests. Now they seem to be gnawing more and more at the foundation of the ecosystem – the calcareous algae reef. The researchers led by Douglas Rasher from the Bigelow Laboratory for Ocean Sciences in East Boothbay have now devoted a study to this development.
To shed light on how the ecosystem has evolved in the past, the researchers conducted analyzes similar to studying tree rings. As they explain, the algae build up a new layer of lime every year, which forms growth bands – similar to the annual rings of trees. These tapes also archive whether and how much sea urchins have grazed each year, the researchers explain. As part of the study, they analyzed samples from the reefs using the uranium-thorium method to enable the strata to be dated and thus to develop a chronology of the reef development. These investigations were carried out at the GEOMAR Helmholtz Center for Ocean Research Kiel.
The results show that the feeding load by the sea urchins has increased significantly in the course of the atmospheric changes and the increase in water temperature. The small populations of sea otters were apparently only a cofactor, because the animals were almost exterminated because of their fur in the 18th and 19th centuries. “During the fur trade, the Clathromorphum reefs survived, although there were probably many sea urchins,” says Rasher. “In the meantime, however, the situation has changed drastically. Our study shows that sea urchin feeding on the reefs has been much more dangerous in recent years due to the looming effects of climate change, ”says Rasher. “The warming and acidification of the oceans makes it difficult for calcifying organisms to produce their shells, in this case the protective skeleton. The key species Clathromorphum is now very susceptible to grazing. At the same time, the number of sea urchins has increased significantly. It’s a devastating combination, ”explains Rasher.
Tipping point exceeded
In order to gain more information, the researchers also examined the interaction between calcareous algae reefs and sea urchins in the laboratory. In doing so, they created controlled water conditions as they existed in pre-industrial times, currently exist and could shape the sea at the end of the century. After three months, the calcareous reef samples and the sea urchins were brought together to assess how sea urchin grazing changes in connection with the different water temperatures and acidity levels.
The researchers found that grazing under current conditions is about 35 to 60 percent more intense than under pre-industrial circumstances. Given the possible future conditions, the destruction rates then increased again by around 20 to 40 percent. The results thus confirm that climate change has recently enabled sea urchins to break through the algae ‘s defenses and push the system beyond a critical tipping point, the researchers sum up. “These long-lived reefs are now disappearing before our eyes – we are facing a collapse that could take place in the order of decades,” says Rasher.
Sea otter protection could help
According to him, the finding that the effect is based on the interaction between the loss of sea otters and climate change gives cause for hope. Reducing greenhouse gas emissions is, of course, a fundamental goal. However, the acute problem could be contained by promoting sea otters. “In the case of this ecosystem, restoring sea otter populations would bring many environmental benefits,” says Rasher.
Something similar may also apply in other cases: “It is well documented that humans influence the earth’s ecosystems by changing the climate on the one hand and decimating large predators on the other. But the combination of such processes is rarely studied. The study now suggests that there may be an opportunity to manage large predators in ways that can help slow the rate at which climate change is affecting our natural ecosystems, ”said Rasher.
Source: Bigelow Laboratory for Ocean Sciences, GEOMAR, specialist article: Science, doi: 10.1126 / science.aav7515